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Re: Can of worms

This isn't a real 64 bit solution, but it may give you some of the win you are looking for with 64-bit in exchange for a bit of work:

If you want to access large data structures that are larger than 4GB on a 32-bit machine (e.g. a G4), one way to do that right now is to lay out the entire data structure on disk. Then you use mmap to map in segments of it as needed, keeping below the 2 GB limit. You wont be able to see the whole thing at once, but it is a way of managing very large chunks of data in a 32-bit app and using it as if it was memory.

Now, on G5, the story is a little bit better. Virtual Memory is already 64 bit. Furthermore, VM is backed up by a RAM cache which is also 64 bit. Thus, there is a chance that with continued access you will eventually end up with your data set in RAM and not actually have to go to disk when you touch more than 4 GB of your data file. Thus, the disk paging penalty that would normally apply on a G4 might not be there on a G5. (Note: YMMV, I haven't actually tried this.) So that may be one way to ship an app that uses data sets >4GB that will work on on G4 and G5 and could see a win from more RAM on G5.

Clearly, most people want 64 bit pointers. On G5 (speaking of hardware now) this is a separate execution mode. When a process is running in 64 bit mode, the meaning of some of the assembly language instructions changes. Thus it is actually pretty much a separate ABI. You would have to go back and fix up your code and then compile a separate 64-bit clean version to get code to run in that mode. (Note: Such code will never run natively on a G4. There is no hardware support for it.)

On the software side, the MacOS X.3 Panther kernel currently doesn't support user apps running in this mode. Also the various libraries you will probably need are not written to use that mode on that OS. I can't comment on when or if this software limitation will change, but users, particularly in the scitech community, have very clearly communicated that they wish this feature.

Ian
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